This invention relates generally to high-Z inter-scintillator materials and a method of manufacturing such inter-scintillator reflectors, where the high-Z inter-scintillator material acts as a high density x-ray absorber.
Solid state detectors for computed tomography (CT) imaging use scintillators to convert x-rays into scintillation radiation which itself is converted to an electrical signal with a photodiode. Detector arrays are typically comprised of scintillator pixels separated by an inter-scintillator material used to pipe the scintillation radiation towards the diode. The materials used as the inter-scintillator material are typically highly reflecting at the scintillation radiation wavelengths emitted by the scintillator in order to collect a large fraction of the scintillation radiation at the diode.
Appropriate inter-scintillator materials include high refractive index solid materials such as TiO2 formed in a castable low index medium such as an epoxy. One drawback of such a system is the darkening of the inter-scintillator material when it is struck by a dose of x-rays commonly used in CT imaging. A typical dose over the life of the detector is 1 Mrad. This darkening results in lower reflectivity and less efficient collection of the scintillation radiation, and thus a lowering of the sensitivity of the x-ray detector.
Furthermore, the darkening is often not uniform over the entrance face of the detector. This lack of uniformity in darkening can result in image degradation if the detector is not properly calibrated. In addition to the inter-scintillator material itself, the diode below the reflector is also sensitive to radiation and must be protected from the x-ray beam.
Current CT detectors use a collimator assembly to protect the inter-scintillator material from damage by x-rays. This assembly consists of tall tungsten plates aligned perpendicular to the plane of the x-ray fan beam. This assembly is primarily used to minimize scattered x-rays from reaching the scintillator, but is also used to protect the inter-scintillator material between pixels from the x-rays. For multi-slice CT, where the detector is segmented in the direction parallel to the fan beam, wires are used to protect the reflector and diodes. These wires are strung between the deep plates in grooves machined in the plates.
The manufacturing of such a two dimensional collimator with plates and wires is complex. The separate construction of the collimator with protective wires and the scintillator/reflector body requires accurate alignment of these devices during construction of the complete detector. This alignment cannot be done optically since the inter-scintillator material between the scintillator pixels is obscured by reflector material covering the top of the pixels (“surface reflector”). Therefore, either x-ray alignment or rigorous dimensional tolerances must be used to ensure that the reflector material is aligned with the protective wires.